Implement GetP4InfoManager() method (#317)

- Added a GetP4InfoManager() method to TdiTableManager, to provide
  diagnostic access to its P4InfoManager object.

Signed-off-by: Derek Foster <derek.foster@intel.com>

stratum/hal/lib/tdi/tdi_table_manager.h

@@ -105,6 +105,12 @@ class TdiTableManager {
       OperationMode mode, TdiSdeInterface* tdi_sde_interface,
       TdiTargetFactory& tdi_target_factory, int device);
 
+  // Returns a read-only pointer to the P4InfoManager object.
+  // For medicinal purposes only.
+  const P4InfoManager* GetP4InfoManager() const {
+    return p4_info_manager_.get();
+  }
+
  private:
   // Private constructor, we can create the instance by using `CreateInstance`
   // function only.